1. Field of the Invention
The present invention relates to a technique of performing compression coding on a document image constructed with plural pages, and generating one file.
2. Description of the Related Art
In recent years, electronic image data read and acquired by a scanner from an original paper document is kept (stored) in a computer and reused.
However, image data (particularly color image data) has a large data size. In the case in which image data is stored on a server provided on a network, a network traffic problem occurs. Furthermore, to store the data, a large-capacity storage device is required.
To address these issues, a number of image data compression techniques have been proposed. If image data is compressed at an extremely high compression rate, image quality deteriorates significantly, causing a problem in which the text portion that is considered important in many business documents becomes illegible.
To solve this problem, the following techniques are known: the technique of changing the compression method in accordance with a characteristic or attribute of the image data (e.g., Japanese Patent Application Laid-Open No. 10-51642 and No. 10-215379), and the technique of extracting layer structures of an image and performing efficient compression for each of the layers (Japanese Patent Application Laid-Open No. 2000-306103).
According to these techniques, one sheet of document image data can be compressed to image data of a small data size, while maintaining reasonable image quality. Assume that one page of uncompressed image data can be compressed as small the amount as 1/10. In this case, a storage device having a certain capacity can store and manage 10 times as large the document volume. Although the compression rate can further be increased, an extremely high compression rate results in unclear text in a decoded document and makes the texts difficult to read.
According to the techniques proposed so far, document images are stored and managed in units of sheet and are independent of each other, thus having no cause-and-effect relationship.
Meanwhile, a document prepared for presentation used in an office conference or the like is often constructed with plural pages, and many of them are generated by a PC. In the case of such documents prepared for a presentation, a background image is often provided to each page for better appearance.